This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-265356, filed Sep. 20, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to, for example, an image formation apparatus, such as a digital copying machine, which scans a laser beam emitted from a semiconductor laser source across a photosensitive drum to form an electrostatic latent image on the drum and then applies a developer to the latent image to form an image and more specifically to a method of stabilizing the emission level of the semiconductor laser source used in the image formation apparatus.
In general, image formation apparatuses, such as digital copying machines, which use a semiconductor laser source as a light source are arranged to detect the emission level of the laser source at image formation time by means of a photodiode built in or outside the laser source and stabilization-control the emission level through the use of the output current of the photodiode (such control is generally called APC: Auto Power Control).
The APC is generally performed on the emission level of the semiconductor laser source at the time of image formation. In this system, no current is applied to the semiconductor laser source when no image is formed. In another APC system, the power is controlled by supplying the semiconductor laser with a constant amount of current that is below a threshold current which is a point of change at which the semiconductor laser makes the transition from the LED emission (i.e., spontaneous emission) region to the laser oscillation region.
It is well known that, when a current corresponding to the emission level at image formation time, for example, a current in the form of a rectangular pulse, is rapidly applied to the semiconductor laser source which has not been supplied with any current, an overshoot, called relaxation oscillation, occurs in the rising portion of a light output of the semiconductor laser.
With semiconductor lasers that emit light of wavelengths in the range of 630 to 690 nm using AlGaInP, called red semiconductor lasers, it is known that not only the relaxation oscillation is involved, but the relationship of thermal conductivity among the materials that construct the laser requires several microseconds until the light output reaches a desired intensity level even if a rectangular pulse of current is applied. In image formation under such conditions, disadvantages arise in that the tip of a fine line formed along the direction in which a laser beam is swept becomes too fine, and the tip portions of a solid image become blurred.
There is also likely to arise a problem that noise is radiated to outside through an interconnection pattern or wire harness on a control board on which the drive circuit of the semiconductor laser is mounted because a drive current in excess of 50 mA is controlled at a high frequency corresponding to a picture signal.
Next, a description is given of a case where the semiconductor laser has been supplied in advance with a constant current and a current corresponding to a signal indicating whether to form an image or not is superimposed on that constant current.
The semiconductor laser causes spontaneous emission until a certain fixed current flows, but not laser action. When the fixed current is exceeded, the semiconductor laser performs laser action. The constant current, referred to as a threshold current, varies from device to device or from manufacturing lot to lot. The threshold current increases with increasing environmental temperature.
When a current corresponding to a signal indicating whether to form an image or not is superimposed on the constant current, therefore, there arises a problem that, depending on the environmental temperature, the waveform of light output of the semiconductor laser may vary. Specifically, in the event that, at a low temperature, the threshold current lowers below the constant current, the semiconductor laser will start laser action with the constant current. That is, laser action is performed even when no image is to be formed, which causes a problem in which an unwanted tonor image is formed.
Conversely, when the threshold current increases at a high temperature to become much higher than the constant current, there arise problems that the effect of adding the constant current is reduced, an overshoot occurs in the light output waveform, and several microseconds are taken for the light output to reach a desired intensity level.
At to noise, it is inevitable that a phenomenon will occur by which radiation noise increases with increasing temperature.
It is an object of the present invention to provide an image formation apparatus and a method of stabilizing the emission level of a semiconductor laser source used therein which permit stable emission intensities of the laser source to be always obtained both at the time no image is formed and at the time an image is to be formed regardless of variations in ambient temperature and consequently an image uniform in density to be formed.
It is another object of the present invention to provide an image formation apparatus and a method of stabilizing the emission level of a semiconductor laser source used therein which, even with variations in operating temperature (ambient temperature), prevents radiation noise and the response time of the semiconductor laser source from varying and thus permits stable light output waveforms with short rise and fall times to be obtained.
In order to achieve the above objects, according to one aspect of the present invention, there is provided an image formation apparatus for forming an image on an image carrier by scanning an imaging region on the image carrier and a non-imaging region with a laser beam emitted from a semiconductor laser source, comprising: a detector for detecting the emission level of the laser source; a first emission level control unit which controls a drive current supplied to the laser source on the basis of the result of detection by the detector so that the laser source emits light at a predetermined spontaneous emission level at the time no image is formed by the laser source; and a second emission level control unit which controls the drive current applied to the laser source on the basis of detection by the detector so that the laser source emits light at a first laser emission level at the time an image is formed by the laser source.
The first and second emission level control units are adapted to control the drive current when the laser beam is scanning the non-imaging region. The first emission level control unit includes a first drive unit which, during a first time interval, drives the laser source with a current which corresponds to the sum of a first drive current corresponding to the spontaneous emission level and a second drive current having a fixed value to thereby allow the laser source to emit light at a second laser emission level, and the second emission level control unit includes a second drive unit which, during a second time interval, drives the laser source with a third drive current corresponding to the first laser emission level to thereby allow the laser source to emit light at the first laser emission level.
As a result, the emission intensities of the laser source at both the imaging and non-imaging times can be controlled to desired values at all times regardless of variations in ambient temperature. In addition, even if the operating temperature (ambient temperature) varies, the response time required for the semiconductor laser source to rise/fall remains unchanged; thus, light output waveforms with good rise and decay times can be obtained at all times.
According to another aspect of the present invention, there is provided an image formation apparatus for forming an image on an image carrier by scanning a laser beam emitted from a semiconductor laser source across the image carrier through an optical system, comprising: a detector for detecting the emission level of the laser source; a first adjust unit which adjusts variations in the detecting efficiency of the detector; a first emission level control unit which controls a drive current supplied to the laser source on the basis of the result of detection by the detector so that the laser source emits light at a predetermined spontaneous emission level at the time no image is formed by the laser source; a second emission level control unit which controls the drive current applied to the laser source on the basis of detection by the detector so that the laser source emits light at a first laser emission level at the time an image is formed by the laser source; and a second adjust unit which compensates for variations in attenuation characteristics of the optical system.
Variations in the detection efficiency of the detector for detecting the emitting conditions of the semiconductor laser and variations in the optical characteristics of the optical system can be compensated for. Thus, the emission level control is performed more precisely.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter